The broad concept of impressing a phase (or frequency) modulation onto a fiber optic cable used for telecommunications is known in the art. Modulating means is affixed externally to the fiber and coupled thereto without breaking the fiber. An illustrative application includes a single fiber cable disposed within (or around) a telecommunication highway, and a plurality of modulating means may be attached to impress the status of a number of machines. The status from each device is modulated onto the cable, and a phase/frequency monitor decodes and displays the desired data.
The following U.S. patents are examples of systems employing fiber optic modulation:
______________________________________ 4,077,700 Camphausen 4,086,484 Steensma 4,068,191 Zemon et al 4,002,896 Davies et al 3,781,092 Sussman 3,756,690 Borrelli et al 3,625,589 Snitzer ______________________________________
U.S. Pat. No. 4,002,896 to Davies et al. discloses a number of modulating means along a fiber optic telecommunications system to impress a phase modulation (see col. 2, lines 18-39) onto the signals carried by the fiber. In FIG. 2 a piezoelectric plate is affixed by an adhesive to the outside of fiber to effect the modulation. FIGS. 6 and 7 (col. 4, line 41) show an optical fiber and a covering of piezoelectric material 86. On the inside and outside are conductive layers 84 and 88. Leads 90 are connected to the conductive layers.
The present invention is a broader concept of impressing the electric field or electromagnetic radiation directly upon the piezoelectric element which covers or is attached to the optical fiber. The piezoelectric element, e.g. PVF.sub.2, i.e. polyvinylidene fluoride, lithium niobate, zinc oxide, or lead zirconate-lead titanate, is in contact with the optical fiber and when electromagnetic radiation is impressed on the element, it produces a strain in the element and in the fiber. An optical phase change in the fiber is thus produced due to the changes in the index of refraction as well as length.
The Davies et al. patent teaches use of a telecommunication system which uses optical fibers with a source of coherent light, such as a laser. The sensor consists of a data unit coupled to the highway by means of electrically driven piezoelectric film (FIG. 6, col. 4, line 51). The film is energized to provide a data input to the highway which is then detected. The fiber detects the change in three ways--a change in length, a change in diameter and a change in refractive index. The diameter change is very small (0.2%).
The concept is that light from a laser and electromagnetic radiation can be modulated by a driven piezoelectric data unit and then demodulated. The communication highway is planned for fixed elements of data to be modulating the carrier and then detected. It always has to be electrically coupled, and then detected. The electrically coupled or driven piezoelectric material is physically moved to change length, diameter and/or refractive index.
U.S. Pat. No. 3,681,092 to Sussman et al. also discloses a monitoring system wherein the data being monitored is modulated onto optical fibers. The disclosure, however, is largely in terms of a brute force (chopper) modulator, and only a terse suggestion of analog modulators is noted in column 5, lines 48-62.
U.S. Pat. No. 4,068,191 to Zemon et al. teaches the modulation of light within fiber optic waveguides by externally applied means. In this instance, the fiber is acoustically modulated in response to corresponding RF energy applied to the modulating means.
With respect to fiber optics, modulators per se, which are externally applied without breaking the fiber, are taught in U.S. Pat. No. 3,625,589 to Snitzer, which discloses a piezoelectric modulator; U.S. Pat. No. 3,756,690 to Borrelli et al., which teaches the use of a magnetic field modulator (of a special fiber); U.S. Pat. No. 4,077,700 to Camphausen, which teaches the use of CCD to create an electric field for modulating, and U.S. Pat. No. 4,086,484 to Steensma, which discloses an acoustic transducer for optical fiber modulation.
Measurements of optical fiber modulation are frequently accomplished with interferometers. A discussion follows on the kinds of interferometers used which will aid in clarifying the disclosure of the present invention.
Much of the early work with optical fiber interferometric sensors was done using the Mach-Zehnder interferometer, wherein fiber elements replaced air paths using microscope objectives to focus the light into the fibers on the input ends and monitoring with one (or two) photodetectors the central bright spot of the interference pattern formed by combining the expanding beams from the output ends of the two fibers. U.S. Pat. No. 4,238,856 to Bucaro et al. describes, however, a Fabry-Perot type interferometer sensor.
At the present time, all-fiber interferometers are coming into use. These include the Michelson, Mach-Zehnder and Fabry-Perot interferometer configurations. A fiber pig-tail is attached to a solid state laser and coupled to the input end of a fiber. The beam splitters are replaced with fiber-to-fiber couplers, which are one of several types of existing elements where the cores of two fibers are brought close together, e.g. by etching away or lapping down part of the cladding, so that light is transferred from one core to the other. The couplers split the laser output into two equal portions and recombine the light in the two fiber arms, which go on to directly couple into the photodetectors. A signal processor operates on the electrical output from the two photodetectors, each of which is capable of detecting the optical phase shift of one beam or optical path with respect to the other.
In connection with this invention, we use MachZehnder or other types of fiber interferometers with a sensing element in one or both of the fiber arms. The latter configuration acts as a gradiometer to measure spatial variations of electric fields, i.e. a gradient type sensor for electric fields.